1. Field of the Invention
The present invention relates to a signal reproducing method and apparatus capable of irradiating a light beam on a magneto-optical recording medium having a multilayered structure, moving magnetic walls of a recording mark in a reproduction layer by using the temperature gradient of a temperature profile with respect to a magnetic domain without changing recorded data in a recording layer, detecting a change in the plane of polarization of the reflected light of the light beam, and reproducing the recording mark smaller than the diffraction limit of the light beam.
2. Related Background Art
One example of rewritable high-density recording media is a magneto-optical recording medium by which information is recorded by writing a magnetic domain in a thin magnetic film by using thermal energy of a semiconductor laser and read out by using a magneto-optical effect. Recently, it is being increasingly demanded to further increase the recording density of this magneto-optical recording medium to obtain a large-capacity recording medium. The linear recording density of an optical disk such as a magneto-optical recording medium largely depends upon the laser wavelength of a reproducing optical system and the numerical aperture of an objective lens. That is, the diameter of a beam waist is determined when a laser wavelength .lambda. of a reproducing optical system and a numerical aperture NA of an objective lens are determined. The detectable band of a spatial frequency during reproduction of a recording mark is at most 2NA/.lambda., and this is an optical diffraction limit. Therefore, to realize a high density by using a conventional optical disk, it is necessary to shorten the laser wavelength of a reproducing optical system and increase the NA of an objective lens. However, improvements of the laser wavelength and the numerical aperture of an objective lens also have their limits. For this reason, it is being attempted to develop a technique which increases the recording density by improving the arrangement of a recording medium or a read method.
For example, Japanese Laid-Open Patent Application No. 06-290496 has proposed a signal reproducing method and apparatus which operate as follows. That is, signal recording is performed in a recording holding layer of a multilayered film including a reproduction layer and the recording holding layer, which are magnetically coupled with each other, and having different Curie temperatures. Magnetic walls of a recording mark in the reproduction layer are moved by using a temperature gradient caused by irradiation of a heating light beam without changing the recorded data in the recording holding layer. The reproduction layer is magnetized such that the directions of magnetization are aligned in a nearly entire area of a reproducing light beam spot. A change in the plane of polarization of the reflected light of the reproducing light beam is detected, and thereby the recording mark smaller than the diffraction limit of the reproducing light beam is reproduced.
FIGS. 5A and 5B show the relationships between a large circular heating beam, a small circular recording/reproducing beam in the heating beam, and an oval T.sub.s isothermal line corresponding to the beam moving velocity during reproduction in this method. When a recording mark shown in FIG. 4D is scanned and reproduced, the reproduction signal is a rectangular signal as shown in FIG. 4E. That is, it is possible to reproduce a recording mark with a period smaller than the diffraction limit of the light beam without decreasing the amplitude of a reproduction signal. Accordingly, a magneto-optical medium and a reproducing method capable of greatly improving the recording density and the transfer rate can be obtained.
FIG. 3 shows the arrangement of the prior art. Referring to FIG. 3, a magneto-optical disk 1 includes a substrate 2 made from glass or plastic, a magneto-optical recording medium 3 having three or four layers and adhered to the substrate 2, and a protective layer 4 formed on the magneto-optical recording medium 3. The magneto-optical recording medium 3 can move magnetic walls of a recording mark in a reproduction layer by using a temperature gradient caused by irradiation of a light beam without changing recorded data in a recording layer, align the directions of magnetization in a nearly entire area of a reproducing spot, detect a change in the plane of polarization of the reflected light of the light beam, and reproduce the recording mark. The magneto-optical disk 1 is supported by a spindle motor by means of, e.g., magnet chucking and can be rotated around a rotating shaft.
Elements 5 to 17 are parts constituting an optical head for irradiating laser light onto the magneto-optical disk 1 and obtaining information from the reflected light. The element 6 is a condenser lens, and the element 5 is an actuator for driving the condenser lens 6. The element 7 is a recording/reproducing semiconductor laser with a wavelength of 680 nm, and the element 8 is a heating semiconductor laser with a wavelength of 1.3 .mu.m. The elements 9 and 10 are collimator lenses. The element 11 is a dichroic mirror for transmitting 100% of 680-nm light and reflecting 100% of 1.36-.mu.m light. The element 12 is a beam splitter. The element 13 is a dichroic mirror which prevents 1.3-.mu.m light from entering a signal detecting system. The dichroic mirror 13 does not transmit 1.3-.mu.m light and transmits 100% of 680-nm light. The element 14 is a .lambda./2 plate, the element 15 is a deflection beam splitter, the elements 17 are photosensors, and the elements 16 are condenser lenses for the photosensors 17. A differential amplifier 18 differentially amplifies signals focused and detected in accordance with the deflecting directions.
In the magneto-optical recording/reproducing apparatus with the above arrangement, laser beams emitted from the recording/reproducing and heating semiconductor lasers 6 and 8 and having wavelengths of 680 nm and 1.3 .mu.m are irradiated on the magneto-optical disk 1 via the collimator lenses 9 and 10, the dichroic mirror 11, the beam splitter 12, and the condenser lens 6. The condenser lens 6 is so controlled by the actuator 5 as to move in a focusing direction and a tracking direction to sequentially focus the laser beams on the magneto-optical recording medium 3 as a recording/reproducing laser and a heating laser. The condenser lens 6 also tracks along guide grooves formed on the magneto-optical disk. Additionally, a light beam system of the 1.3-.mu.m light is so designed as to be smaller than the aperture diameter of the condenser lens 6. Therefore, the NA of the 1.3-.mu.m light is smaller than that of the 680-nm light which is condensed through the entire aperture. Accordingly, as shown in FIGS. 5A to 5C, the heating spot of the heating beam has a long wavelength and a small NA and hence has a larger diameter than that of the recording/reproducing spot of the recording/reproducing beam. Consequently, as indicated by the moving direction of a light beam, a desired temperature gradient which gradually drops from the characteristic maximum temperature with respect to the medium temperature can be formed in a recording/reproducing spot region on the moving medium surface.
The optical paths of the laser beams reflected by the magneto-optical disk 1 are changed in the direction of the polarizing beam splitter 15 by the beam splitter 12. These light beams are focused on the photosensors 17 by the condenser lenses 16, in accordance with the polarity of magnetization in the magneto-optical recording medium, via the dichroic mirror 13, the .lambda./2 plate 14, and the polarizing beam splitter 15. Since the 1.3-.mu.m light cannot transmit through the dichroic mirror 13, the heating light beam is intercepted, and the 680-nm light alone propagates after that. The differential amplifier 18 differentially amplifies the outputs from the photosensors 17 and outputs a magneto-optical signal.
A controller 20 inputs, of course, a recording signal to be recorded and also receives, e.g., the rotational speed, recording radius, and recording sector of the magneto-optical disk 1 as input information and outputs the recording power of the semiconductor laser 7 and the recording signal, thereby controlling an LD driver 19 and a magnetic head driver 24. The LD driver 19 drives the semiconductor lasers 7 and 8 and controls desired recording power, reproducing power, and heating beam power in this prior art.
A magnetic head 23 applies a modulated magnetic field to a laser irradiated portion of the magneto-optical disk during recording. The magnetic head 23 is arranged to oppose the condenser lens 6 with the magneto-optical disk 1 interposed between them. In recording, the recording/reproducing semiconductor laser 7 is driven by the LD driver 19 to irradiate the recording laser power by DC light. At the same time, the magnetic head 23 is driven by the magnetic head driver 24 to generate a magnetic field having a different polarity in accordance with a recording signal. Also, this magnetic head 23 moves in the radial direction of the magneto-optical disk 1 in synchronism with the optical head. In recording, the magnetic head 23 sequentially applies a magnetic field to laser irradiated portions of the magneto-optical recording medium 3, thereby recording information.
Recording and reproducing operations will be described below with reference to FIGS. 4A to 4E. FIG. 4A shows a recording signal. FIG. 4B shows a recording power of the semiconductor laser 7 for generating a recording/reproducing beam. FIG. 4C shows a modulated magnetic field generated by the magnetic head 23. FIG. 4D shows a recording mark formed on the magneto-optical disk 1. FIG. 4E shows a reproduction signal obtained from the differential amplifier 18. When a recording signal as shown in FIG. 4A is to be recorded, the laser power is set at a predetermined recording power at the start of a recording operation, and a modulated magnetic field based on the recording signal is applied. By these operations, the recording mark string shown in FIG. 4D is formed in the cooling process of the recording medium. Note that hatched portions represent magnetic domains having a direction of magnetization corresponding to the recording mark described in this specification, and halftone portions represent magnetic domains having an opposite direction of magnetization.
In a reproducing operation, as shown in FIGS. 5A to 5C, the heating beam from the heating semiconductor laser 8 performs heating until a temperature condition by which a magnetic wall in the reproduction layer of the magnetic wall movement moves is obtained. Under this temperature condition, an isothermal line of the recording medium temperature T.sub.s as a principal condition by which a magnetic wall starts moving exists in both before and after the beam along the beam moving direction. That is, one magnetic wall moves backward in the beam moving direction, and the other magnetic wall moves forward in the beam moving direction. Therefore, when the reproducing beam is arranged in the front portion along the beam moving direction as shown in FIG. 5A, only a magnetic wall movement signal from the front can be detected. Also, when the reproducing beam is arranged in the rear portion along the beam moving direction as shown in FIG. 5B, only a magnetic wall movement signal from the rear can be detected. FIG. 5C shows the temperature characteristic of the magnetic domain with respect to the temperature of the magneto-optical medium. Even a recording mark whose recording density is smaller than the optical diffraction limit as described above can be reproduced by using the temperature gradients on the two sides of the maximum temperature point.
In either of the cases shown in FIGS. 5A and 5B, the reproduction signal shown in FIG. 4E can be obtained by reproducing the recording mark string as shown in FIG. 4D by using the recording/reproducing beam. This reproduction signal in FIG. 4E faithfully reproduces the recording signal in FIG. 4A, so the information can be reproduced. In this method, as shown in FIGS. 5A to 5C, all magnetized states contained in the reproducing beam are the same. Therefore, the reproduction signal is a rectangular signal as shown in FIG. 4E, so a recording mark with a period smaller than the optical diffraction limit of the light beam can be reproduced without decreasing the amplitude of the reproduction signal. This realizes a magneto-optical medium and a reproducing method capable of greatly improving the recording density and the transfer rate.
In the above prior art, however, the use of the heating laser beam increases the number of optical parts and the number of adjustment steps during assembly of the apparatus. That is, the use of the two lasers increases various costs. To solve the problem of these increases in the costs, it is necessary to make a single light beam function as both a heating beam and a reproducing beam.
If the heating beam is omitted, however, as shown in FIG. 1, the maximum temperature point in a temperature rising region formed by the reproducing light beam is present in the reproducing light beam. Accordingly, a synthetic signal: h(t)=f(t)+.alpha..times.f(t-.beta.) of a signal: f(t) generated when a magnetic wall moves from the front end of a magnetic movement critical temperature region, which is formed by the reproducing light beam, to the maximum temperature point and a signal: .alpha..times.f(t-.beta.) generated when a magnetic wall moves from the rear end of the critical temperature region to the maximum temperature point is detected as a reproduction signal.
For example, when a recording signal string as shown in FIG. 2A is to be recorded and reproduced, recording marks are read out through states indicated by (I) to (IV) in FIG. 2B as a reproducing beam moves, and a reproduction signal as shown in FIG. 2C is obtained. As described above, the reproduction signal in FIG. 2C is a superposed signal of reproduction signal (I) obtained by magnetic wall movement from the front of the reproducing beam and reproduction signal (II) obtained by magnetic wall movement from the rear of the reproducing beam. In this case, therefore, the recorded information cannot be reproduced with a sufficient margin by a conventional method (FIG. 6) which performs binarization by a slice level of the median value of a repeating reproduction signal of the shortest mark.